The present invention relates to position control apparatuses which drive an object and control its position, such as optical apparatuses which control an optical element.
In positional servo controls which cause the position of an object to follow a target position, a controller generally has a control function which, in the case where the difference between the detected position of the object and the target position is within a predetermined range, assumes that the position of the object reaches the target position and thereby does not drive the object. In other words, the controller provides a dead band of control.
FIG. 6 shows a block diagram of a position control apparatus having a dead band. A target position signal set by a target position setting part 601 and a position detection signal that is an output of a position signal converter 606 are input to a subtracter 608. A deviation signal obtained by the subtracter 608 (a signal showing the difference between the target position signal and the position detection signal) is input to a dead-band providing part 602. The dead-band providing part 602 sets the value of the deviation signal to zero in a case where the value of the deviation signal from the subtracter 608 is within a predetermined dead band. Thereby, the control state becomes a non-controlled state in which the servo control is not performed.
FIG. 7 shows the input-output relationship of the dead-band providing part 602. The output of the dead-band providing part 602 becomes zero when the deviation amount shown by the input deviation signal (positional deviation) is smaller than the dead band. The dead-band providing part 602 generates an output proportional to the positional deviation when the positional deviation is larger than the dead band.
FIG. 8 shows the relationship between the target position signal and the position detection signal in the position control apparatus having the dead band. A normal position control (normal drive) is performed outside the dead band, and thereby the position detection signal follows the target position signal. However, inside the dead band, since the output value of the deviation signal is forced to be zero, the position control is not performed, and thereby the last position before the position control is stopped is held. However, an overshoot may occur due to the inertia of an object 609 to be controlled.
The deviation signal from the dead-band providing part 602 is input to a gain/phase compensator 603. The gain/phase compensator 603 performs a gain and phase compensations on the deviation signal to generate a driving signal for driving an actuator. The generated driving signal is input to an linear actuator 605 via a driver 604. The linear actuator 605 thereby is driven to move the object 609 to be controlled to a target position.
The position of the object 609 to be controlled is detected by a position detector 607. The output of the position detector 607 is converted into a signal having the same dimension as that of the target position signal, and fed back to the subtracter 608. The control with such a dead band has a purpose of preventing a busy control feeling, generation of abnormal noise due to frequent driving of the object, and premature deterioration of the control performance.
On the other hand, positional control with high accuracy is often required in the field of precision apparatuses such as optical apparatuses. Therefore, an art enabling positional control with high accuracy has been proposed in Japanese Patent application Laid-Open No. H01(1989)-162279, in which a dead band is provided in a state that an object is away from a target position to some extent, and the dead band is eliminated in a state that the object is near the target position.
However, in the art proposed in Japanese Patent application Laid-Open No. H01(1989)-162279, there still remains a problem that the above mentioned busy control feeling and abnormal noise are generated and the control performance deteriorates near the target position since the dead band is eliminated. Therefore, the above-mentioned problem occurs even when the positional control with high accuracy is not required.